install prereqs after installing py3

Author: vimanyu

.github/workflows/cpp-packaging.yml

@@ -156,14 +156,14 @@ jobs:
         with:
           path: sdk-src
 
-      - name: install prerequisites
-        run: sdk-src/build_scripts/ios/install_prereqs.sh
-
       - name: Setup python
         uses: actions/setup-python@v2
         with:
           python-version: 3.7
 
+      - name: install prerequisites
+        run: sdk-src/build_scripts/ios/install_prereqs.sh
+
       - name: build sdk
         run: |
           python sdk-src/scripts/gha/build_ios_tvos.py -b firebase-cpp-sdk-ios-tvos-build -s sdk-src